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Abstract

In the present work, the characterization of cobalt-porous silicon (Co-PSi) hybrid
systems is performed by a combination of magnetic, spectroscopic, and structural techniques.
The Co-PSi structures are composed by a columnar matrix of PSi with Co nanoparticles
embedded inside, as determined by Transmission Electron Microscopy (TEM). The oxidation
state, crystalline structure, and magnetic behavior are determined by X-Ray Absorption
Spectroscopy (XAS) and Alternating Gradient Field Magnetometry (AGFM). Additionally,
the Co concentration profile inside the matrix has been studied by Rutherford Backscattering
Spectroscopy (RBS). It is concluded that the PSi matrix can be tailored to provide
the Co nanoparticles with extra protection against oxidation.

Keywords:

Background

The development of hybrid materials is a current topic of research with many potential
applications in several fields, including optoelectronics, catalysis, and biomedicine.
Concretely, the hybridization of semiconductors with ferromagnetic material such as
cobalt, iron, and nickel gives the possibility to obtain materials that combine semiconducting
and magnetic properties
[1]. Moreover, the continuous progress in nanotechnology during the last decades has
led to a large availability of techniques for the fabrication and characterization
of nanometric structures with controlled composition and dimensions, resulting in
nanostructures with very specific properties and several functionalities
[2]. In fact, multifunctional metal-based nanostructures have received a great deal of
attention during the past few years given their special properties and potential applications
in many scientific and technologic fields, including biomedicine
[3]. In this sense, the conjugation of magnetic-semiconductor hybrid nanosystems has
allowed manipulation of local spin in spintronics
[4,5] and the fabrication of high sensitive magnetic sensors
[6]. Regarding porous semiconductors such as porous silicon (PSi), they present additional
advantages including high surface area and high surface reactivity
[7].

This work aims at studying the oxidation state and crystalline structure of cobalt
nanoparticles (NPs) embedded into porous silicon, resulting in Co-PSi hybrid structures.
Co has been infiltrated into the PSi matrix by electrochemical techniques
[8]. The suitability of PSi to host Co NPs grown by electroinfiltration has been evaluated,
and both the magnetic and structural properties of the hybrid structures have been
studied. The role of the porous matrix protecting Co against oxidation has been evaluated
by infiltrating the Co NPs into PSi layers with different morphologies. The chemical
and structural states of the Co NPs have been determined by combining highly selective
and sensitive characterization techniques such as X-Ray Absorption Spectroscopy-synchrotron
(XAS-synchrotron) and Rutherford Backscattering (RBS).

Methods

Preparation of porous silicon

Porous silicon (PSi), both as single layers and multilayers, was obtained from high
conductivity (0.01-0.02 Ωcm), p-type, silicon wafers. The anodization in 1:2 (volume)
HF:ethanol solutions (from commercial HF 48% (w/v) in water, Sigma-Aldrich) was carried
out in a homemade Teflon® electrochemical cell, with a Pt reference electrode. The
Si wafers were galvanostatically etched under illumination from a 100 W halogen lamp.
Single layer PSi were obtained at 100 mA/cm2 for 100 s, resulting in layers of about 10 microns. Different current densities were
set for obtain different porosities from 40 to 120 mA/cm2 in case of multilayer configurations (Figure
1). Two different porosity gradients were chosen to electrochemically grow Co NPs into
PSi: A negative gradient (more porous towards the surface) and a positive gradient
(more porous towards the substrate). In both cases the current densities applied were
in four steps: 20, 40, 60 and 80 mA/cm2. After etching, the Si/PSi substrates were rinsed in ethanol and dried with N2.

Electroinfiltration of Co

Electroinfiltration of Co nanoparticles into PSi single- and multi-layers was carried
out in a electrochemical bath with a Watt’s solution at RT (CoSO4·7H2O 0.2 M, CoCl2 0.05, H3BO3 0.4 M, Na-Saccharine 25 g/l H2SO4 1 mM). Boric acid was used as a buffer, to avoid pH fluctuations, sodic saccharine
as crystallization catalyzer and sulfuric acid to decrease the pH of the solutions
below 3. A well parameterized pulsed current process was used to allow the nucleation
of metal nanoparticles into the pores of pulses of 40 mA/cm2 and 10 s. An important parameter in pulsed mode is the equilibrium time or rest time,
between pulses, set at 60 s; estimated by observing the dynamics of the voltage versus
time curves (not shown). A overall view of the whole process of PSi single/multi layer
fabrication and Co electroinfiltration is summarized in Figure
2.

Figure 2.PSi is obtained by electrochemical anodization of highly conductive p-type Si wafers
in HF:ethanol solutions (1:2). The nucleation of Co inside the matrix is achieved by electroinfiltration of Co in
the porous matrix by a pulsed mode.

Characterization

Field emission scanning electron microscopy (FESEM) images were obtained in a XL 30S-FEG (PHILIPS). No metallization was required to
observe the samples. Samples for cross section observation were prepared according
to previously optimized protocols for mechanical and ion bean milling
[9]. TEM/STEM characterization was carried out using a Jeol JEM 3000F with HAADF (High
Angle Annular Dark Field) system included (300kV).

Alternating gradient field magnetometry

Magnetic characterization was performed using a Micromag Model 2900 Alternating Gradient
Magnetometer System (Princeton Measurements Corporation). Nine measurements were taken
for each sample. Measurements were carried out applying a magnetic field from -100mT
to 100mT, a time pass of 100 ms, and a field pass of 800μT.

Rutherford backscatering spectroscopy

Micro analytical techniques were used to obtain in-depth elemental information. A
Cockcroft-Walton tandem accelerator located at Centro de Micro-Análisis de Materiales
(CMAM, Universidad Autónoma de Madrid, Spain) was used for Rutherford Backscattering
Spectroscopy (RBS). RBS was performed with a 3.050 MeV He+ beam (incidence angle was 75° with respect to the surface normal). RBS was acquired
by using silicon surface barrier detectors placed at scattering angle of 170°. A 13 μm
thick mylar foil was placed in front of the detector on the forward scattering angle
to stop the He forward scattered particles and filter the H recoils. All RBS experiments
were performed in vacuum (pressure lower than 10E-5 mbar). All spectra were simulated
using the SIMNRA code
[11] to obtain the element in-depth composition.

Results and discussion

PSi single layers

Preliminary structural studies of Co-PSi hybrid structures are carried out by Transmission
Electron Microscopy (TEM) in a milled sample of single layer Co-PSi. As Figure
3 shows, Co ions infiltrate inside the PSi matrix and form spherical nanoparticles
(NP) into the pores. In this case, nanoparticles of circa 5 nm are formed. Scanning
TEM (STEM) images plus EDX analysis of these Systems (Figure
3) allow the identification of both elements in the NP.

images of typical Co-PSi structures showing Co NPs into the PSi matrix; (c) STEM image
of typical Co-PSi structures, showing the Co NPs in a brighter tone than the matrix,
composed by a lighter element (Si and O).

Magnetic properties of single layer PSi-Co are evaluated by Alternating Gradient Field
Magnetometry (AGFM). Samples electroinfiltrated at an increasing number of cycles
with Co, from 5 to 20 cycles, are evaluated. Saturation magnetization values normalized
to mass increase with increasing number of cycles in the studied range of cycles.
Results point to the possibility of tailoring the magnetization of Co-PSi by controlling
the amount of Co inside the PSi matrix (Figure
4). This is achieved by setting the number of electroinfiltration pulses during the
synthesis process. Coercivity magnitude of the material depends on the amount of Co
in the hybrid material: less concentrated systems present lower coercivity values
[4,6]; systems with higher Co concentration present higher values. This feature can be
due to the appearance of inter-particle correlations inside the matrix or by the increase
of the size of particles
[12] favoured by an increasing density of NPs into the PSi matrix.

Figure 4.AGFM magnetization curve for Co-PSi systems with increasing Co concentration inside
the matrix, obtained from increasing number of electroinfiltration pulses: 5, 10 and
20. Demonstrating the control of the matrix filling with Co. (inset) Axis detail showing
coercivity dependence of the magnetic material for increasing amount of internalized
Co nanoparticles.

PSi multilayers

Once studied the magnetic potential of Co-PSi systems, the protective effect of the
PSi matrix against the oxidation of Co is determined. Magnetic properties of Co strongly
depend on the oxidation state and therefore determine the possible applications of
these hybrid systems
[13]. For this purpose a multilayer PSi is fabricated and infiltrated with Co. Two multilayer
configurations are made, as Figure
1 shows, one with a negative porosity gradient (Co-PSi-n) and other with a positive
one (Co-PSi-p). These systems are studied by XAS
[14-16] (Figure
5). XANES spectra (Figure
5a) of both systems show clear differences in oxidation state. Co and Cobalt oxides
references are obtained for comparing and adjusting the experimental spectra to known
materials. From the fitting of the experimental data to references it can be observed
that in Co-PSi-n systems, Co atoms are mainly in metallic state (close to 90% at.)
and the remaining 10% at. is in the form Co3O4. Besides, studying Co-PSi-p systems shows Co to be composed by a mix of Co (40%at.),
CoO (50%at.) and Co3O4 (10%at). The EXAFS spectra of the Co-PSi show the element environment and lattice
structure in the Fourier space (Figure
5b). Comparing the EXAFS spectra of the Co-PSi systems with the reference it is straightforward
to identify similarities of that of Co-PSi-n with the spectrum of metallic Co. The
similarities are clearer if we obtain the transform the spectra to the real space
by obtaining the Radial Distribution Function (RDF, Figure
5c). In this figure the spacial distribution of neighbours around the scattered element
is represented and gives an overall situation of the short range environment of the
Co. In Figure
5c the similarities of CoPSi-n with metallic Co are clearer. In case of CoPSi-p, EXAFS
spectra has more common features with CoO than with Co3O4: position and shape of the RDF in the first neighbour matches with the O in the CoO,
but the second one matches with the position of Co atoms in the metallic Co, confirming
the observations in XANES spectrum.

The elemental concentration profile of these systems is studied by Rutherford Back
Scattering (RBS). RBS spectra and elemental profiles of both systems are depicted
in Figure
6. Figure
6a,b, show the raw RBS spectra of the CoPSi-n and CoPSi-p samples, respectively, and
the simulated spectra with the SIMNRA software
[11]. A reference of Ta-Ti alternated multilayers was used to perform the energy calibration
of the experimental system. By using this reference, a fine adjustment of the set
up parameters was achieved. In the analyzed PSi systems, the porosity degree does
not contribute significantly to the measured oxygen amount. The void (or porous) part
of the samples does not produce backscattering nor energy loss, and a porous material
is indistinguishable of a bulk one by this technique. This is a reason to not be able
to determine porosity profiles directly by RBS. In this respect, other authors have
carried out these calculations by filling pores with hydrocarbon solvents like pentane
to obtain porosity profiles in PSi multilayers
[17,18]. In these cases the filling substance can be quantified and an indirect measurement
of the porosity can be obtained.

Figure 6.(a,b) RBS spectra of Co-PSi-n and Co-PSi-p systems, respectively showing experimental
data (dots), and simulation (lines). (c and d) Elemental concentration profiles of Co-PSi-n and Co-PSi-p, respectively obtained
from simulations of the spectra shown in a and b.

From the raw spectra (Figure
6a,b) we observe a clear difference in the profiles. The feature in the higher energies
region corresponding to the Co presents a localized broad peak in case of Co-PSi-p
and an extended region for Co-PSi-n. In a first approximation there should exists
differences in the Co distribution along the layer in both configurations. Both spectra
present the Si (1600–1700 keV) and O (~1000 keV) edges with subtle differences that
require finer calculations. By using the fitting software it has been determined that
in Co-PSi-n structures, Co is uniformly distributed along the multilayer, and the
oxygen concentrates mainly near the surface. This means that metallic Co is hosted
in deeper zones of the multilayer and Co3O4 is concentrated in outer zones of the matrix. For Co-PSi-p systems, Co penetrates
to less deep zones and the O concentration keeps fairly constant along the multilayer.
This is due both to the presence of O bubbles inside the matrix and the coexistence
of a mixture of Co oxides. Figure
6c,d summarize the elemental in-depth profiles of each systems, CoPSi-n and CoPSi-p,
respectively, obtained from the simulations of the experimental spectra.

A typical cross section of a CoPSi-n system is observed by TEM to study the distribution
of the Co deposits along the PSi structure. A representative image is presented in
Figure
7. In this image, the distribution of the Co NPs inside the PSi matrix can be clearly
identified and also the porous structure of the PSi layer. An image in the STEM mode
(inset to Figure
7), which enhances the contrast with the differences in atomic weight, facilitates
the identification of Co inside the Si matrix. Again, Co NPs of circa 5 nm and spherical
shape are observed inside the PSi matrix, with a roughly uniform distribution of them
inside the porous structure.

Figure 7.Cross section TEM images of a sample Co-PSi-n showing the Co NPs inside the porous
matrix. The inset is a magnification in STEM mode highlighting the atomic weight of the elements.

Conclusions

In this work the protective effect of the matrix on the oxidation of electroinfiltrated
Co nanoparticles into PSi has been proved. Moreover, a selected multilayer configuration
in the matrix allows minimizing the formation of Co oxides that forms an oxidation
profile in the whole layer depending on the porosity gradient of the PSi template.
The magnetic properties of such Co-PSi systems suggest the possibility to control
the magnetization of these hybrid materials by controlling the amount of infiltrated
Co. Nevertheless, the influence of the PSi porosity type and degree in the oxidation
of the hosted Co need to be further characterized. Future work in this sense should
be performed in order to identify the physico-chemical properties of the PSi matrix
that minimizes the oxidation in such hybrid materials.

Acknowledgements

We acknowledge the European Synchrotron Radiation Facility for provision of synchrotron
radiation facilities and the MEC and Consejo Superior de Investigaciones Científicas
for financial support (PE-2010 6 0E 013). We would like to thank the BM25-SpLine staff
for the technical support. This work has been supported by MICINN through projects
FIS-2008-06249, MAT2009-14578-C03-02, MAT2008-06858-C02-01, and MAT2008-06858-C02-02,
as well as Comunidad de Madrid, project NANOBIOMAGNET (S2009/MAT-1726) and European
project MAGNIFYCO (Contract NMP4-SL-2009-228622).Technical support from ICTS Centro
Nacional de Microscopia Electrónica (UCM, Madrid) is gratefully acknowleged.